Earth in Space Introduction Origin of the Universe The Solar System The Earth & Sun Near Earth Objects Impact Hazards Beware Flying Rocks Summary We travel together, passengers on a little space ship, dependent on its vulnerable reserves of air and soil; all committed for our safety to its security and peace; preserved from annihilation only by the care, the work, and, I will say, the love we give our fragile craft. Adlai Stevenson We have only one planet. If we screw it up, we have no place to go. J. Bennett Johnston Introduction • Ancient civilizations believed in a geocentric universe in which the Sun revolved around Earth. • Early astronomers such as Copernicus, Kepler, and Galileo advanced the concept of a heliocentric universe with the Sun at its center. • Our home planet has a unique position in our solar system, providing us with liquid water and sufficient heat energy to sustain life. • Geological processes on Earth are driven by energy from the interior of the planet or from solar radiation. • The future of life on Earth is threatened by a collision with near-Earth objects such as asteroids and comets. Earth's Orbit Ancient civilizations observed the Sun rising in the east and setting in the west and inferred that the Sun revolved around Earth in a geocentric (Earth-centered) orbit. The Greek philosopher Aristotle believed Earth was at the center of the universe and that the visible planets (Mercury, Venus, Mars, Jupiter, Saturn) and stars revolved around the Earth. Aristarchus, another Greek philosopher, calculated the relative size of Earth and the Sun and concluded that it was more probable the that Earth revolved around the massive Sun in a heliocentric (Sun-centered) orbit. However, his interpretation would go unheeded for nearly 1800 years. The geocentric model became increasingly complex nearly five centuries after Aristotle to account for more-detailed Figure 1. Relative positions of Sun, Earth, and Mars in models of heliocentric (top) and geocentric (bottom) orbits. Earth and Mars both orbit the Sun in the heliocentric model. Earth makes nearly two orbits of the Sun during a single Mars orbit. The geocentric model required that Mars followed a path that described a small circle as it revolved around Earth. 2 observations of planetary motion. Ptolemy updated Aristotle’s work to account for apparent reversals in the orbits of the visible planets. The new model concluded that planets orbited Earth along circular paths but would also follow a route around a smaller circle (Fig. 1). The Ptolemic system was accepted without any serious challenge for over a thousand years but additional celestial observations required that the geocentric system be further modified, making it increasingly complex and unwieldy. Nicolaus Copernicus (1473-1543) became an advocate for the heliocentric universe in the sixteenth century. Copernicus inferred that the planets revolved around the Sun in circular orbits and determined the relative distance of the planets from the Sun based on their reversals of motion. In addition, he recognized that Earth must spin on its axis once per day. Although his reinterpretation of the known solar system was able to simplify some of the complexity of the Ptolemic system, Copernicus still had the planets describing secondary orbits along small circles and was unable to offer any unassailable proof that the heliocentric view was superior to the geocentric interpretation. Copernicus published his ideas in his book, On the Revolutions of the Heavenly Orbs, in 1543. Figure 2. View of Earth from the About a century later, German astronomer Johannes Kepler Moon by Apollo 8, (1571-1630) modified the ideas of Copernicus to conform to the first manned more-detailed observations. Kepler discovered that the planets craft to orbit the had elliptical, not circular, orbits and that the speed of Moon, December, planetary motion decreased with distance from the Sun. Kepler 1968. Image was the first astronomer to calculate the length of time it would courtesy of NASA take for planets to complete an orbit. Italian mathematician Galileo Galilei (1564-1642), a contemporary of Kepler, introduced technology into cosmic exploration when he built an early telescope in 1609. Galilei used his telescope to make the first observations of the cratered landforms on the Moon's surface, the larger moons of Jupiter, and the phases of Venus (changes in the appearance of the planet as it orbited the Sun). Kepler's ideas coupled with Galileo's observations were sufficient to convince skeptics that the heliocentric system accurately portrayed the relative motions of the sun and planets. Finally, Isaac Newton discovered the force that held the planets in their orbits around the sun - gravity. He formulated one of the universal laws of nature, the law of gravitation, "every body in the universe attracts every other body." 3 Earth in Space Although we were able to explain Earth's position in space, the unique nature of our planet was not driven home until we were able to look at our home from the outside. The astronauts aboard the Apollo 8 spacecraft were the first to glimpse our home planet from space (Fig. 2). While orbiting the Moon on Christmas Eve 1968, the crew read the first 10 verses of Genesis during a broadcast to Earth. At the end of the reading Frank Borman closed communications with ". Merry Christmas, and God bless all of you, all of you on the good earth". For many back home, those early views of the planet from the inky darkness of space illustrated the unique wonders of the fragile environment we share on spaceship Earth. In this chapter we seek to introduce you to the reasons why that natural environment exists and to a potential threat to its future. Figure 3. Earth The chapter is divided into six sections; the first three examine viewed from space. Image courtesy of Earth's position in space and the remainder discuss the NASA. potential hazards associated with the collision of an asteroid with Earth. The Origin of the Universe takes us on a journey through time and space to examine how scientists think the universe began and to explore some of the far corners of the Cosmos. We will place Earth and the Sun in the context of the much larger universe and learn if there are other systems of planets and stars out there that might harbor life. We follow that with a closer look at our own Solar System where we compare Earth to our neighboring planets. We exist because our home, this good Earth, is perched 150 million kilometers from the Sun, close enough to have liquid water to sustain life, and far enough away to moderate the Sun's heat (Fig. 3). The solar system examines the fortunate set of conditions that makes life on our home planet possible while our nearest neighbors orbit the Sun as barren rocks. The geological processes that operate on Earth draw their energy from the decay of radioactive materials in the interior of the planet and from solar radiation absorbed on or near the surface. We take a closer look at the structure of Earth's interior in the section on the Solar System, while the Earth & Sun examines how the distribution of solar radiation on Earth's surface regulates the length and order of the seasons and provides the energy for the operation of the biosphere, hydrosphere, and atmosphere. We will also examine how the elements of the earth system are linked by cycles that transfer energy and resources between different parts of the system. The interaction of solar radiation with our atmosphere generates a beneficial greenhouse effect that has contributed to 4 a flourishing biosphere. We will introduce the linkage between atmospheric composition, solar radiation, and global climate in this section. Death from the Sky $100: Price Michelle A 12-year old red Chevrolet Malibu Classic would seem like Knapp paid her an odd choice to appear in the American Museum of Natural grandfather for her History. The presence of the car seemed even more surprising Malibu Classic when you noticed the gaping hole that passed from the trunk through the gas tank. However, it is this hole that gave this $10,000: Selling price of Michelle's car particular Malibu Classic its significance. The hole formed on a following the 1992 fall evening when a 12 kg (27 pound) meteorite smashed meteorite impact through the car and embedded itself in Marie Knapp's driveway in Peekskill, New York. The car belonged to her daughter $69,000: Selling price Michelle and quickly became a scientific icon among the of the meteorite community of meteorite hunters willing to pay top dollar for these flying space rocks. The Peekskill meteorite represents just one of thousands of objects that collide with Earth each year. Some are large enough to reach the surface of the planet relatively unscathed, but most of these cosmic visitors burn up harmlessly in the atmosphere. The second half of the chapter reviews the current state of knowledge about the potential for collision with such near-Earth objects (NEOs). Past impacts by large NEOs are thought to have resulted in a widespread extinction approximately 66 million years ago that wiped out the dinosaurs and a more recent explosion in the last century that felled 2,100 square kilometers (840 square miles) of Siberian forest. The section on Near-Earth Objects examines where these objects come from and discusses their potential for collision with Earth. The evidence for past impacts and the potential consequences of such an impact are discussed further in the Impact Hazards section. We will learn that NEOs routinely strike our planet and that approximately one impact per century has the potential to cause widespread destruction equivalent to a major natural hazard.